The public switched telephone network (PSTN) is a synchronous transfer mode (STM) network in which time division multiplex (TDM) switches are interconnected by transport links commonly referred to as "trunks". Each trunk supports a plurality of "channels" which are time slots used by individual calls. Although the STM network is reliable and robust, the demand for voice and other voice-band data services is taxing the capacity of current network facilities.
Consequently an interest has developed in using alternate facilities to complete voice grade connections. Many local exchange carriers (LECs) are experiencing tandem network congestion and are seeking economical ways to grow their tandem networks. Inter-exchange carriers (IECs) would like to consolidate their voice and data traffic on a single multi-service network. The currently most attractive alternate facility is a network which operates in an asynchronous transfer mode (ATM) protocol. ATM has a now matured to an extent that it provides a viable alternative to STM facilities.
ATM uses a fixed data packet size of 53 octets called a "cell". A cell includes a header of 5 octets and a payload of 48 octets for transferring user information. There are known methods for converting STM voice and voice-band call data to ATM cells and vice versa.
Call setup and control in the PSTN is commonly effected using an out-of-band signaling network known as a common channel signaling network. Most of the North American PSTN is equipped to operate with a common channel signaling protocol called Signaling System 7 (SS7). ATM networks, however, use a different signaling protocol in which signaling messages are transported through the network in cells like those used for carrying payload data. The signaling systems of the PSTN and ATM networks are therefore incompatible and STM calls cannot be transferred directly to or from an ATM network.
Methods and apparatus for transferring voice and voice-data services over ATM networks are known, however. One ATM network architecture designed for that purpose is described in U.S. Pat. No. 5,568,475 entitled ATM NETWORK ARCHITECTURE EMPLOYING A COMMON CHANNEL SIGNALING NETWORK, which issued Oct. 22, 1996 to Doshi et al. In the ATM network architecture described by Doshi et al, each switch in the ATM network is equipped with a signal processor that is capable of sending and receiving common channel signaling messages. The signal processor translates STM trunk identification information into ATM Permanent Virtual Circuit (PVC) information to permit STM calls to be transported through the ATM network using virtual trunks. While this ATM architecture provides an option for a migration of PSTN voice services to an ATM protocol, it appears to suffer from certain drawbacks. First, the architecture requires that all ATM switches be enabled with SS7 signaling capability and that the SS7 network overlay the entire ATM network. Second, the use of PVCs in the ATM network has the potential to tie up ATM resources unnecessarily, preventing those resources from being used for other purposes, even during off-peak calling hours.
There therefore exists a need for a method and apparatus for transferring STM calls in a multi-service ATM network which requires only a small investment in infrastructure and ensures efficient use of ATM network resources.
Simply transferring STM calls through an ATM network to relieve congestion in the PSTN is not in itself enough, however. In North America, the current call setup rate is 4,000,000 calls per busy hour and PSTN usage is expected to double over the next five years. Despite the current call volume, call setup within an STM node requires only 20 msec, and service is delivered with 99.999% availability.
There therefore exists a need for a method and apparatus for transferring STM calls in a multi-service ATM network which ensures that calls transferred through the ATM network are rapidly set up so that current PSTN service levels are sustained.